The present invention relates to methods and apparatus for controlling the degree of saturation in a chamber and more particularly to the control of solvent vapor concentrations in a curing oven.
Techniques for curing solvent borne coatings on materials passed through curing ovens have typically included supplying massive air flows through the oven to maintain the solvent vapor concentration of the oven atmosphere below explosive levels. However, as these extremely large air flows remove heat from curing ovens, which typically operate at temperatures between 200.degree.-600.degree. F., considerable quantities of energy in the form of heat are required to cure such solvent borne resin coatings. More recently, it has been proposed to discard the foregoing traditional approach toward curing of solvent borne resin coatings by introducing an inert gas into a curing oven at a flow rate sufficient to maintain oxygen levels below limits of combustibility, i.e. 4%. This technique, which is described in U.S. Pat. No. 4,150,494 and which is assigned to the assignee of the present invention, enables the efficient recovery of solvent vapor by permitting relatively high degrees of solvent vapor saturation (concentrations) in the curing oven which in turn facilitates the condensation of solvent vapor exhausted or removed from the oven. By the technique described in this patent, solvent can be recovered by condensing the withdrawn solvent vapor against a refrigerant such as liquid nitrogen which in turn is vaporized and is supplied to the oven as an inert gas.
In addition to the aforementioned technique for inerting curing ovens, it has also been determined that inert gas flows to such ovens must be controlled in order to assure that desired, low oxygen levels are maintained in the oven and that excessive quantities of inert gas are not consumed or lost to ambient atmosphere. Such a technique for controlling inert gas flows to a curing oven is described in U.S. Pat. No. 4,223,450 which is also assigned to the assignee of the present invention. However, neither of the aforementioned techniques, although relying upon high degrees of solvent vapor saturation in curing ovens, fully enable the degree of saturation of such vapor to be controlled upon variations of oven temperatures and/or the degree of solvent loading with respect to a given coating. It will be understood that as essentially continuous material bearing coatings to be cured is passed through a curing oven, the amount of solvent per unit length of coating may change from one section of such material to another and consequently, the rate at which solvent vapor is introduced into the oven atmosphere may vary substantially during operation of the curing oven. In these latter circumstances and, upon certain temperature variations, an oven atmosphere which exhibits a high degree of solvent vapor saturation may result in a condensation of such solvent vapor in the oven itself or associated piping or incomplete curing of the resin coating. Although oxygen levels are maintained below limits of combustibility, this condensed solvent is not recovered and its presence in an oven is considered to be a potentially hazardous condition and one which should be avoided.
Consequently, there is a need to develop techniques for controlling the degree of solvent vapor saturation in curing ovens or the like to assure that condensation of such vapors in the oven is avoided but concurrently enabling high solvent vapor concentrations to exist to render economic the recovery of solvent vapors withdrawn from the oven. Again, the higher the degree of solvent vapor saturation in a withdrawn or exhaust stream, the less refrigeration and hence energy is required to recover the same by way of condensation and it is these dual objectives of avoiding condensation in the oven yet enabling high degrees of solvent vapor saturation to exist therein which are sought to be attained by the present invention.